(i) Field of the Invention
The present invention relates in general to rotor assemblies for use in rotating machines, and more specifically to an improvement in the attaching of magnets to the rotor core of a rotor assembly.
(ii) Description of the Related Art
Machines powered by electric motors often use motors having permanent magnet rotor assemblies. The typical rotor assembly of this type has a plurality of permanent magnets spaced around the periphery of a rotor core. The rotor core has a cylindrical exterior surface and the magnets have arcuate shapes with concave interior surfaces and convex exterior surfaces. The magnets are held in place relative to the rotor core by adhesives between the magnet interior surfaces and the rotor core exterior surface.
In the process of attaching the magnets to the rotor core with adhesives, the thickness and uniformity of the adhesive layer greatly determines the shear strength of the adhesive. It is desirable to control the thickness and uniformity of the adhesive layer to ensure adequate adhesion. It is also desirable when attaching the magnets to the rotor core to have the magnets maintain their shape and to not deform, as will be discussed in more detail below.
When attaching the permanent magnets to the rotor core with adhesives, the magnets usually have a relief or recess ground into the interior surface of the magnet to provide a recess within the surface for the adhesive. The adhesive is then applied to the recess in the magnet interior surfaces and the magnets are positioned on the exterior surface of the rotor core. A clamp or other retaining means is then tightly secured around the magnet exterior surfaces to hold the magnets in place on the rotor core until the adhesive has cured.
The use of a clamp has drawbacks. When the magnets are clamped to the rotor core, excessive tightening of the clamp, variation in rotor size, and variation in magnet thickness can all cause the clamp to deform the magnets and compress the gaps formed between the recesses in the interior surfaces of the magnets and the exterior surface of the rotor core expelling some of the adhesive from the gaps. The compression of the gaps can change the thickness of the adhesive layers to less then the required thickness. The reduced thickness of the layer of adhesive can cause premature failure of the adhesive. Additionally, when the clamp is removed (after the adhesive layer has cured), the magnets will decompress and attempt to return to their normal/relaxed (non-compressed) states. The gaps will also try to return to their original dimensions and the adhesive layer will resist the gaps returning to their original dimensions. As a result, the magnets will exert a tensile force on the adhesive layer and thus form a weakened bond between the magnets and the rotor core. The tensile force on the adhesive layer and the weakened bond can result in magnets coming loose from the rotor and the failure of the motor.
Furthermore, the need to have a relief ground in the magnets adds to the cost to manufacture the rotor assembly.
The present invention overcomes the shortcomings of prior art rotor assemblies by providing a rotor assembly that uses a clamp and an adhesive layer to attach the magnets to the rotor core while minimizing the potential variability in the thickness of the adhesive layer and the tension on the adhesive layer caused by excessive tightening of the clamp, variation in rotor size, and/or variation in magnet thickness.
The improved rotor design does not require the use of recesses in the interior surfaces of the magnets for the adhesive. Rather, the improved rotor design uses one or more radial protrusion(s) on the cylindrical exterior surface of the rotor core to form a recess on the rotor core for the adhesive. Magnets are positioned around the periphery of the rotor core on the radial protrusion(s) which prevent the interior surfaces of the magnets from contacting the rotor core exterior surface and support the interior surfaces of the magnets in a radially outward spaced relation from the rotor core exterior surface. The protrusion(s) thereby form a radial spacing between the interior surfaces of the magnets and the rotor core exterior surface. The adhesive is positioned in the radial spacing formed by the protrusion(s) instead of in a recess in the interior surfaces of the magnets as is done in the prior art. The radial protrusion(s) support the interior surface of the magnets and minimize and/or prevent the magnets from deforming radially inward when being held in place by a clamp or other retaining means while the adhesive cures. The protrusion(s) also help maintain a uniform radial spacing between the interior surface of the magnet and the rotor core exterior surface so that the layer of adhesive is uniform and of the required thickness.
The protrusion(s) can be of a variety of sizes and shapes that will maintain the magnets in a radially spaced relation from the core exterior surface. For example, the protrusion(s) can be bumps, ribs, spikes, or the like and can be pointed, rounded, squared, etc. The protrusion(s) can also extend along the exterior surface of the rotor core. For example, the protrusion(s) can extend along the core exterior surface circumferentially, axially, diagonally, etc. Additionally, the protrusion(s) can be spaced around the rotor core exterior surface in a variety of arrangements, such as equally circumferentially spaced positions of the protrusions around the rotor core exterior surface. However, it is preferred that the protrusion(s) be spaced around the rotor core so as to facilitate the manufacturing of the rotor cores, the assembly of the rotor assemblies, and the support of the magnets.
Each protrusion can be arranged on the rotor core exterior surface to be in contact with only a single portion of a magnet, such as a mid portion of the magnet, or arranged to be in contact with multiple portions of the magnet, such as opposite end portions and a mid portion of the magnet. Additionally, each protrusion can be in contact with and support multiple magnets and more than one protrusion can be in contact with and support the same magnet. A variety of configurations and arrangements of the protrusion(s) on the exterior surface of the rotor core that radially space the interior surface of the magnets from the exterior surface of the rotor core may be employed.
The use of the protrusion(s) allows use of a clamp or other retaining means to hold the magnets around the rotor core while the adhesive cures without excessive tightening of the clamp, variation in the rotor size, or variation in magnet thickness causing the clamp to deform the magnets, expel some adhesive from the radial spacing and/or cause a tensile force to be exerted on the adhesive layer when the clamp is removed (after the adhesive has cured). The improved rotor design thereby prolongs the life of the rotor assembly and overcomes the disadvantages and limitations of the prior art.
In another aspect of the invention, there are two rotor cores. The two rotor cores are similar and are axially stacked end to end to form a larger rotor assembly. The two rotor cores of the larger rotor assembly are held axially together by being press fit on a rotor shaft. The rotor cores each have axially opposite first and second ends. The first ends each have one or more projections extending axially outwardly from the first ends and the second ends each have one or more notches extending axially inwardly from the second ends. The notches and projections are complementary so that the projection(s) on the first end of one rotor core engage in the notch(es) on the second end of the other rotor core when the two rotor cores are axially stacked. The engagement of the projection(s) in the notch(es) axially and circumferentially aligns the two rotor cores. The protrusion(s) on the exterior surfaces of both rotor cores are arranged to be aligned when the two rotor cores are axially stacked so as to provide the same support on each of the rotor cores for the magnets positioned around the periphery of the two rotor cores.
In still another aspect of the invention, a method of assembling a rotor assembly is provided. The method comprises the steps of providing a rotor core such as that described above. Providing one or more magnets and positioning the magnet(s) on the protrusion(s) of the rotor core so that the magnet(s) are supported in a radially outward spaced relation from the core exterior surface by the protrusion(s). The radially outward spaced relation thereby defines a radial spacing between the rotor core exterior surface and the interior surface of the magnet(s). A connection is provided in the radial spacing that holds the magnet(s) to the core exterior surface. The connection is provided by applying an adhesive in the radial spacing that holds the magnet(s) to the rotor core exterior surface. The method also comprises placing a clamp around the magnet(s) and the rotor core to hold the magnet(s) in place on the protrusion(s) while the adhesive is curing. The adhesive is allowed to cure and then the clamp is removed, producing the improved rotor assembly.
These different aspects of the invention overcome the drawbacks of the prior art. The use of a protrusion(s) to support the magnet(s) in a spaced relation from the core exterior surface(s) prevents or at a minimum helps to minimize the deformation of the magnet(s) when being held around the rotor core(s) by a clamp or other retaining means. The minimal deformation or absence thereof minimizes or prevents a tensile force from being exerted on the connection holding the magnet(s) to the core exterior surface(s) when the clamp or other retaining means are removed. Additionally, the protrusion(s) provide a generally uniform radial spacing between the magnet(s) and the core exterior surface(s) and thereby aids in achieving the desired uniform thickness of adhesive holding the magnet(s) to the core exterior surface(s).